Welding of high-tensile steels should be performed under strict control of a preheating/interpass temperature so as to prevent cold cracking of weld beads (weld metals), thus causing reduction in operation efficiency. Steels for use in welded structures have had higher and higher strengths, and weld metals should have higher strengths. Typically, high-tensile steels of HT780 class having a high tensile strength on the order of 780 MPa have been used.
Such increase in strength may tend to cause reduction in cold cracking resistance. To avoid this, weld metals should have better cold cracking resistance. In particular, gas-shielded arc welding using a flux-cored wire is employed because of exhibiting excellent weldability, and a weld metal formed by this welding technique should surely have satisfactory cold cracking resistance.
The cold cracking is probably caused by segregation of diffusible hydrogen at a grain boundary to reduce the grain boundary strength (hereinafter this phenomenon is also called as “hydrogen embrittlement”). Accordingly, an improvement in cold cracking resistance may significantly depend on reduction in diffusible hydrogen.
The improvement in cold cracking resistance of a weld metal requires reduction in susceptibility to hydrogen embrittlement of the weld metal, and various techniques have been proposed from this viewpoint.
Typically, PTL 1 discloses a technique of dispersing a molybdenum carbide (carbide containing Mo) in a weld metal so as to prevent cold cracking, because such molybdenum carbide can satisfactorily trap hydrogen. This technique, however, is not applicable to regular welding of steels, because it requires a special welding technique in which steels are butted to each other, and submerged arc welding is performed from the inner wall, so as to disperse the molybdenum carbide.
PTL 2 proposes a technique of controlling the cooling time during welding operation so as to prevent cold cracking. This technique, however, disadvantageously requires strict operation control according to its chemical composition and suffers from a high work load.
PTL 3 proposes a technique of allowing a weld metal to have a retained austenite fraction of 1% or more so as to prevent cold cracking, because the retained austenite will trap diffusible hydrogen. This technique, however, presupposes double one layer seam welding and is not applicable to regular welding of steels.
PTL 4 proposes a technique of reducing the amount of diffusible hydrogen and suitably controlling the strength and chemical composition of a weld metal so as to improve cold cracking resistance of the weld metal. Even this technique, however, is applicable to limited areas in actual welding operations, because a strength at satisfactory level obtained according to this technique is affected by the chemical composition.
Such proposed techniques are each intended to improve cold cracking resistance, but there more essentially needs improvement in hydrogen embrittlement resistance of a weld metal, because the amount of hydrogen in the weld metal can be increased by various factors in actual welding operations.
High-tensile steels of HT780 class have been more and more applied to weld metals for use in offshore structures. These weld metals require not only hydrogen embrittlement resistance and strength but also low-temperature toughness at high levels so as to endure use in cold climate areas.